For several photographic applications the need exists to have the disposal of a reproduction material which exhibits increased image contrast upon exposure to radiation and subsequent processing. This need originates from the knowledge that image contrast (also called `gradation`) is directly related with the appearance of sharpness. Photographic products showing increased gradation are known therefore to exhibit a higher sharpness and a better quality of reproduced image details.
One way to increase gradation of an emulsion is by doping the emulsion with a metal ion or its complex. The metal ion or complex, also called dopant, is therefore supplied to the silver halide emulsion during precipitation and is incorporated in the internal crystal structure.
Depending on the concentration of dopants in said crystal structure the photographic properties will differ. The ligands of the dopant are also included in the crystal structure and will modify the photographic properties as well.
Generally an increase of gradation by addition of dopants is also accompanied by a decrease of sensitivity further depending on the kind of metal ion, its valency, ligand structure and amount of metal-complex added during precipitation.
This sensitivity decrease can advantageously be used to make less light-sensitive materials which can be handled under safelight conditions. In several graphic art applications these materials are e.g. used for roomlight operations, as in contact printing of halftone film materials where negative or positive copies are made from screened originals by dot per dot reproduction.
The dopants which will be discussed here are characterised by building a deep electron trap in a silver halide crystal lattice. Such trap is called `deep` if following two conditions are fulfilled: the LUMO of the incorporated molecular entity should be at least 0.5 eV below the conduction band of the silver halide crystal, and the trapping life-time at room temperature should be higher than 0.2 seconds (see R. S. Eachus, M. T. Olm in `Cryst.Latt.Def.and Amorph.Mat.`,1989(18)297-313). The LUMO is defined as the `lowest unoccupied molecular orbital` of the related complex which can trap an electron from the conduction band (see D. F. Shriver, P. W. Atkins, C. H. Langford in "Inorganic Chemistry"-Oxford Univ.Press (1990), Oxford-Melbourne-Tokyo).
All patents which will be discussed hereinafter are related with transition metal complexes acting as deep electron traps. From a lot of patents related with this topic U.S. Pat. No. 4,933,272 from McDugle et al should be mentioned. This patent discloses doping agents containing a nitrosyl or thionitrosyl coordination ligand together with a transition metal from the groups 5 up to 10 (also including 10) of the periodic table of the elements. The same author describes dopants containing transition metal complexes with carbonyl-ligands in EP-A 0 415 481. In WO 92/16876 Beavers et al describe a combination of a homogeneously distributed deep electron trap (a transition metal complex with a nitrosyl ligand) and a more `shallow electron trap` (an iridium salt) in the outer shell of the grain. The iridium-center is known to trap photo-electrons temporarily in some cases: at room temperature the electrons will be released in a characteristic time in the order of 0.02 to ca 10 seconds depending on the structure and composition of the silver halide host lattice (see R. S. Eachus and M. T. Olm in the literature cited above). Added in small amounts the said iridium dopant is especially in favour of an improvement of the high intensity reciprocity failure and latent image stability.
Increasing the contrast as much as possible needs a rather high amount of dopant homogeneously distributed over the crystal volume which is also required in order to keep maximum density and to prevent solarization. This has its consequences in using automatic processors where the increasing load of metal complexes by continuous processing asks for special attention in regeneration afterwards. The sensitometric problems as density-loss and solarisation can be solved by a method given by Gingello and Schmidt in EP-A 0 697 619 proposing a non-uniform incorporation of the same dopants. Therefore the metal complexes are built in mainly in the outer region of the crystals.
Most of the emulsions doped with the metal complexes mentioned hereinbefore have a high contrast but are suffering from low sensitivity (as is desired in the case of roomlight-handling). It is however unacceptable for other applications like the reproduction of colour negatives. Colour photography requires perfect matching of the characteristic curves of the blue, green and red sensitive emulsion layers. Control of contrast and of sensitivity for the different emulsions is necessary in order to get a final copy with an acceptable image quality.
Minimizing the sensitivity loss by use of a dopant in order to get a gradation increase is the special object for which a solution has to be found. Several patents are related with problems as loss in sensitivity by doping with metal complexes while a lot of applications on the contrary need a high gradation and a high sensitivity as well. Normally sensitivity or speed decreases when gradation increases and vice versa. Breaking through this `sensitivity-gradation`-relationship is therefore the first object of these patents.
One solution which has been proposed frequently is a chemical ripening of such an emulsion with a labile selenium or tellurium compound. Yoshida e.g. in U.S. Pat. No. 5,348,850 suggests an increase in sensitivity while keeping a high contrast by chemical sensitization with labile Se- or Te-compounds and using a well defined rhodacyanine spectral sensitizer. In this case the Se- or Te-sensitization provides a deep trap at the crystal surface (giving high sensitivity) which is in competition with a deep internal electron trap (giving rise to a high gradation). This way of working normally results in a silver halide light sensitive photographic material which is very susceptible to fog formation. Another possibility to overcome the sensitivity problem can be realised with an adapted processing starting with a high sensitivity where the difference in developability of the latent image of the silver halide emulsion crystals is becoming very low because of the very high activity of the developing agent. This however can easily lead to the formation of fog.
In EP-A 0 552 650 a silver halide material is described which has an increased sensitivity by doping with a polyvalent metal complex. The polyvalent metal compounds used in this case are however not satisfying the conditions of having a DET-activity (DET=deep electron trap) incorporated in the silver halide microcrystals. The result which is realized in an iodide containing silver halide emulsion does not show an increase in gradation. It is also interesting to see that doping with the kind of compounds used in EP-A 0 552 650 in combination with a internal reduction sensitization does not lead to the desired increase of the gradation even if the complexed polyvalent metal ion is incorporated in the center of the grain as is teached therein.
However the problem of sensitivity-loss in doped emulsion crystals can be solved by adding a second type of dopant. This can be a temporary electron trap as IrCl.sub.6.sup.3- or even a more shallow electron trap as Ru(CN).sub.6.sup.4- or Fe(CN).sub.6.sup.4- which can be locally concentrated within a certain area of the crystal volume. This has e.g. been demonstrated by Asami in EP-A 0 423 765 wherein doping with ferri- or ferro-complexes in the outer space area of the AgCl(Br)-crystal gives an increase of gradation and a decrease of loss in sensitivity. In U.S. Pat. No. 5,051,344 Kuno teaches that doping with ferro- or iridium(+3)-ions in the crystal shell of the silver halide emulsion gives a higher gradation and sensitivity. The same effect is described by Oozeki and Ikari in JP-A 6-222487 with a Ru-, Fe- or Ir-complex in the surface area of the crystal.
The activity of deep electron traps is also demonstrated by three patents issued to Bell: U.S. Pat. No. 5,474,888, U.S. Pat. No. 5,480,771 and U.S. Pat. No. 5,500,335 propose the use of an [Os(NO)Cl.sub.5 ].sup.2- complex which is uniformly distributed throughout the crystal or on its surface which gives a very small gradation increase by an equal or little lower sensitivity. For tabular crystals Olm et al (U.S. Pat. No. 5,503,970), Daubendiek et al (U.S. Pat. No. 5,503,971) and Kuromoto et al (US-A 5,462,849) suggest that doping in epitaxilly grown protrusions gives an increase in gradation and sensitivity.
Doping in outer regions of the silver halide crystal volume however may lead to interactions between additives added during chemical sensitization and before coating on one hand and superficially present metal ions at the other hand. These interactions can easily influence preservation properties of the chemically ripened emulsion, thereby asking for new measures in order to prevent such disadvantageous influences.
As discussed hereinbefore film systems were related with preservation of sensitivity while increasing gradation or they were focussed on getting improved sensitivity for the same gradation. To summarize: all these patents were targeting the same goal which will be called hereinafter "getting a better sensitivity-gradation-relationship".